Usually the tubes of such exchangers are attached to tube plates which are very thick and consist of carbon or low-alloy steel. They are clad with stainless steel, for example built up by welding. The tubes are of standard dimensions and of stainless steel. These tubular exchangers working under heat and pressure sometimes pose important corrosion problems.
Amongst the various stainless steels which may be employed for exchanger tubes, ferritic steels of high chrome content have been found to be very suitable. The grade, the composition of which is given below, has shown itself to be extremely interesting in a certain number of cases. For example, it resists particularly well corrosion by a urea-carbamate mixture under the conditions of temperature and concentration which are those in urea strippers. The composition of this grade of steel is as follows:
C .ltoreq. 0.010%, PA1 Cr: 25 to 30% and preferably equal to 26%, PA1 Mo &lt; 4% and preferably equal to 1%. PA1 C .ltoreq. 0.010%. PA1 Cr: 25 to 30%, and preferably equal to 26%, PA1 Mo &lt; 4%, and preferably equal to 1%, PA1 C &lt; 0.020%, PA1 Cr: 20 to 26%, PA1 Ni: 6 to 26%, PA1 Mo: 1 to 5%. PA1 C &lt; 0.030%, PA1 Cr: 18 to 22%, and preferably equal to 20% PA1 Ni: 14 to 18%, and preferably equal to 16% PA1 Mo: 1 to 3%, and preferably equal to 2% PA1 Mn: 4 to 6%, and preferably equal to 5%.
It is unfortunately found that welding of such a grade of steel to the welded steel cladding of tube plates poses problems which have not hitherto been solved.